As public health officials continue to fight the greatest outbreak of measles in the United States since the disease was declared eliminated in 2000, there has been a lot of talk about how to stop its spread.

California might hold an answer.

In 2014, after vaccine refusal helped fuel the Disneyland measles outbreak, the state instituted a series of interventions to increase the number of kindergartners who were up-to-date on their vaccines.

First, legislators passed a bill requiring parents talk to their children’s doctor about the risks of not vaccinating their children for personal beliefs.

Then the state rolled out a campaign teaching educators about conditional admission of non-up-to-date kindergartners, or those who can’t attend if they do not get vaccinated within a certain timeframe.

Then in 2016 the state took its boldest step, banning parents from not immunizing their children due to personal beliefs.

In 2013, so before the trio of interventions, the rate of California kindergartners who were not up-to-date with their vaccines was 9.84%. In 2017, after the interventions had gone into effect, it dropped to 4.87%.

Not only that, the findings revealed a decrease in geographic clusters of under-vaccinated children, which pose a particular threat to herd immunity and can fuel wider outbreaks. Because the measles is so contagious (about 90 percent of people who are unvaccinated and come into contact with the disease will catch it), the percent of people who must be vaccinated in an area in order to sustain herd immunity is very high.

“In the 2012 through 2013 school year there were 3,206 schools located within a cluster, and this decreased to 1,613 schools in the 2016 to 2017 school year,” study author Cassandra Pingali, an ORISE fellow at Centers for Disease Control and Prevention in the Immunization Services Division, told HuffPost.

“By decreasing the clustering and contact of not-up-to-date kindergartners, there is an improvement in community immunity,” she explained.

The Centers for Disease Control and Prevention says that children need two doses of the measles, mumps, and rubella (MMR) vaccine: One when they are between 12 and 15 months and another when they are between the ages of 4 and 6.

The authors of the new study caution about what they call the replacement effect, a phenomenon they documented in another analysis of the California policies they published last month. For a time after the personal belief exemption was eliminated, the rate of non-up-to-date kindergartners increased as medical exemptions jumped instead.

In other words, parents who are vaccine skeptics are likely to try and find other ways to skirt the system.In June, actress Jessica Biel generated a rash of headlines for lobbying against a California bill that would require parents who want a medical exemption to immunization to get approval from a state health officer, cracking down on fake health exemptions.

“Unintended effects such as this need to be carefully evaluated,” Pingali said, “and policymakers should consider such consequences when creatingnew legislation.”